1. Field of the Invention
The present invention relates to a hydraulically operated braking system equipped with a hydraulic booster, for a motor vehicle, and more particularly to improvements in connection with a technique for manually producing a fluid pressure in the event of a failure of an external hydraulic power source or related components.
2. Discussion of the Prior Art
A hydraulically operated braking system with a hydraulic booster for an automotive vehicle is known according to laid-open publication Nos. 60-8144 and 60-38243 (published in 1985) of unexamined Japanese Patent Applications. In the braking system disclosed in these publications, a presser piston, and an input piston operated by a brake operating mechanism are fluid tightly and slidably fitted in a housing so as to define a pressure chamber in front of the presser piston, and a power chamber between the presser and input pistons. The power chamber is supplied with a brake fluid which is delivered from an external hydraulic power source, through a control valve which is operated in response to a relative movement between the presser and input pistons. Thus, a power pressure in the power chamber is controlled. The pressure chamber is connected to brake cylinders of a first braking arrangement, while the power chamber is connected to brake cylinders of a second braking arrangement.
In the hydraulic braking system of the type discussed above, the power pressure applied to the hydraulic booster from the external hydraulic power source through the control valve is directly transmitted to the brake cylinders of the second braking arrangement, as long as the external hydraulic power source, control valve or other related components are normally functioning or intact. At the same time, the power pressure activates the presser piston, whereby a fluid pressure is produced in the pressure chamber and is transmitted to the brake cylinders of the first braking arrangement.
In the event that the power pressure cannot be delivered to the hydraulic booster upon failure of the external hydraulic power source or related components, the fluid pressure in the brake cylinders of the second braking arrangement is not raised by an operating action of the brake operating mechanism. However, when the input piston has been advanced a distance exceeding a predetermined value as a result of the operation of the brake operating mechanism, the input piston comes into abutting contact with the presser piston, whereby an operating force acting on the brake operating mechanism is mechanically imparted to the presser piston through the input piston. That is, the presser piston is activated by the brake operating force, rather than the power pressure of the external hydraulic power source, whereby a fluid pressure is produced in the pressure chamber. Thus, only the brake cylinders of the first braking arrangement are designed operable, irrespective or even in the event of an occurrence of a failure or trouble of the external hydraulic power source or related components.
As described above, the rotating motions of the vehicle wheels corresponding to the brake cylinders of the first braking arrangement can be restrained by the fluid pressures developed in these brake cylinders, even in the event of a failure of the external hydraulic power source or related components. However, the overall braking capability of the braking system for the vehicle as a whole is reduced, since only the first braking arrangement is operable. This undesirably requires the vehicle driver exert an increased operating force on the brake operating mechanism.
To compensate for the reduced braking effect upon failure of the external hydraulic power source, it is proposed to provide the brakes with friction members which have a high coefficient of friction. On the other hand, it is desirable to minimize the coefficient of friction of the friction members, from the standpoint of avoiding squealing noises and vibrations of the brakes. Inspite of this desire, the brakes should employ the friction members with a high friction coefficient, in order to obtain a sufficient braking force even in the event of a trouble as indicated above.